"I believe that we pulled out forces at a time, as you know, when the violence was lower, but we pulled them out on a timeline rather than consistent with the maturation of the government and the security forces," he said.

"The result was that as security declined, all the other stresses have come to bear, to include heavy casualties on the part of the Afghan forces, other nations pulled their forces out as well, and the Taliban was emboldened."

Mattis noted Obama's decision to declare the combat operation over at the end 2014, and then to withdraw U.S. combat firepower from the Afghan military came at a time when it needed it the most, the Examiner reported.

"At one point, when we reduced our forces there, I believe in what was probably in hindsight a misguided application of our forces, we restricted them from using our air support, with some idea we would wean them off the need for it," Mattis said.

"That meant in the mountain country these troops were often fighting at a disadvantage."

Mattis said his new strategy, which will be finished in the coming weeks, will restore the "high ground" by restoring NATO air forces to a position of dominance, the Examiner reported.

According to the Washington Examiner, Mattis is considering adding 3,000 to 5,000 military trainers and advisers to the NATO Resolute Support mission.